Anisotropically conducting adhesive, and process for producing an anisotropically conducting adhesive

ABSTRACT

An anisotropically conducting adhesive includes a thermoplastic base material; and particles which include metal particles and metal ions, which are electrically conductive, and which are finely distributed within the thermoplastic base material below a percolation threshold, wherein the particles are enriched in certain regions under the influence of exposure to at least one of light and heat. A process for producing an anisotropically conducting adhesive includes providing a material comprised of thermoplastic base material in which electrically conductive particles including metal particles and metal ions are dispersed; and exposing the material to at least one of light and heat in predetermined regions in a targeted manner so that a targeted local heating occurs in the exposed regions and an increased mobility of the metal particles and metal ions occurs which is effective to provide a plurality of anisotropically electrically conductive paths having an enriched amount of the electrically conductive particles compared to that of adjacent regions, wherein the metal ions contribute to the formation of the plurality of anisotropically electrically conductive paths by undergoing reduction from the metal ion to the metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an anisotropically conducting adhesive withparticles dispersed in a thermoplastic base material and to a processfor producing an anisotropically conducting adhesive.

2. Description of the Related Art

Anisotropically conducting adhesives are known. In a base material, forinstance a thermoplastic base material, they have finely dispersedconductive particles. The conductive particles are dispersed in the formof particulate powder in the base material. The anisotropicallyconducting adhesives are in sheet form, for instance, and are suited forelectrically conductively gluing together two contacts, for instance twoconductor tracks. The tracks are pressed under pressure and temperatureaction onto the opposed sides of the adhesive, so that the base materialdeformed and the conductive particles distributed in it establish anelectrically conductive connection between the applied tracks. The basematerial performs an adhesive action between the parts having theconductor tracks that are to be glued together.

It becomes clear that the resolution of the anisotropically conductingadhesive, that is, the minimum possible distance between two adjacentelectrically conductive connections, depends on the structure of thedispersed electrically conductive particles. In order that theconductive particles will enable the electrically conductive connectionin only one direction, they must be spaced apart sufficiently from oneanother so that an undesired isotropically electrically conductiveconnection is avoided. This can happen for instance from accidentalcoagulation, or in other words clumping together of the electricallyconductive particles in the base material. As a result, the knownanisotropically conducting adhesives can be used only for contactingcontacts that are located relatively far apart from one another, becauseotherwise an adequately high contact reliability is not achieved.

SUMMARY OF THE INVENTION

The anisotropically conducting adhesive of the invention has particlesdispersed in a thermoplastic base material, which particles are finelydistributed below a percolation temperature, characterized in that theparticles are metal particles and metal ions, and has the advantage overthe prior art that by means of a purposeful coagulation of theelectrically conductive particles, the contact reliability between twoadjacent electrically conductive connections extending through theadhesive is improved. Because the conductive particles are formed bymetal particles finally dispersed below a percolation threshold anddispersed metal ions, and these can be purposefully coagulated to formanisotropically electrically conductive paths, anisotropy of the contactconnection is assured with high certainty. The term percolationthreshold is understood to mean the state in which the conductiveparticles each occupy a random position in the base material, and inwhich there is just barely still no metallically conductive connectionbetween two adjacent particles. Because of the coagulation of theconductive particles to form the electrically conductive paths, theadhesive has regions, advantageously between two adjacent conductivepaths, of improved insulation capacity, since here the number ofconductive particles is reduced as a result of the coagulation into theconductive paths.

As a result of the process according to the invention for producing theanisotropically conducting adhesive, it is highly advantageouslypossible in a simple way to employ process steps, known per se from thestructuring of integrated circuits, in modified fashion. Because athermoplastic base material with conductive particles dispersed in it ispreferably exposed to light via a mask, causing local heating in theexposed regions of the base material, and anisotropically electricallyconductive paths are created in the exposed regions, any arbitrarylayout of anisotropically conductive paths can be created via a largeprocess window, and very high resolution is possible.

In a preferred feature of the invention, it is provided that silvercolloids and silver ions are incorporated in adequate quantity into thebase material, or sufficient silver in a certain form is incorporatedand then ions and colloids are created by suitable treatment. By meansof a suitable reduction of the silver ions in the regions where theanisotropically conductive paths are to be created, it is possible,highly advantageously, to create electrically conductive regions withthe silver, while the regions surrounding the electrically conductiveregions are depleted of silver, so that the anisotropically conductivepaths are attainable with very high resolution, or in other words withan extremely slight spacing from one another. It is preferably providedthat the silver is partially or fully stabilized, preferably by means offormation of a complex, so that in the production of the anisotropicallyconductive paths spontaneous precipitation out of the silver does notoccur, yet such precipitation can take place purposefully in reducedfashion in the region of the desired electrically conductive paths.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous features of the invention will become apparent fromthe other characteristics recited in the dependent claims.

The invention will be described in further detail below in exemplaryembodiments, in conjunction with the associated drawings. Shown are:

FIG. 1, a sectional view through an anisotropically conducting adhesivein the initiate state;

FIG. 2, a sectional view through an anisotropically conducting adhesiveduring a forming operation; and

FIG. 3, a sectional view through an anisotropically conducting adhesivein the contacted state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a substrate 10 is shown on whose surface 12 contact sites 14are disposed. The substrate 10 may for instance be any arbitraryelectronic component. The contact sites 14 form external terminals forcomponents integrated on the substrate 10 or optionally in the substrate10. The contact sites 14 may be formed by conductor tracks, contactpaths, or other elements, for instance. In terms of the geometry of thecontact sites 14, or in other words their geometrical length andgeometrical shape, no special demands are made.

A layer 16 of a hot-melt adhesive is applied to the surface 12 of thesubstrate 10. The layer 16 may for instance be printed, rolled on, orapplied in some other way to the substrate 10. The layer 16 has athermoplastic base material 18, in which electrically conductiveparticles 20 are dispersed, or in other words finely distributed. Theparticles 20 comprise colloidal silver particles 22, on the one hand,which are distributed in the base material 18 below a possiblepercolation threshold. Silver ions 24 are also dispersed in the basematerial 18. The silver particles 22 and the silver ions 24 aredispersed in the base material 18 through the entire volume of the layer16. The silver particles 22 have a diameter in a nanometer range. Duringthe application of the layer 16, the contact sites 14 are surrounded ator on the substrate 10 by the base material 18.

FIG. 2 illustrates the structuring of anisotropically conductive paths26 through the layer 16. To that end, the layer 16 is exposed to UVlight 30 via a mask 28. The mask 28 has openings 32 for this purpose,whose layout preferably corresponds to the contact sites 14 to becontacted in the substrate 10. The UV light 30 thus passes through theopenings 32 of the mask 28 to strike the regions of the layer 16 thatcover the contact sites 14. Within the exposed regions, the UV light 30is absorbed by the silver particles 22, which thus heat up. The heatingof the silver particles 22 causes local heating in the base material 18,so that in these regions the base material 18 becomes fluid. Within thefluid regions thus created, the silver ions 24 become mobile. The silverparticles 22 form condensation nuclei for the silver ions 24, so thatthe silver ions 24 are attracted by the silver particles 22 and aredeposited onto them. The continuing deposition of silver ions 24 causesthe condensation nuclei to grow into the anisotropically conductivepaths 26. Because of the resultant concentration gradients of the silverions 24, even farther-away silver ions 24 are also attracted to theresultant centers of silver particles 22 with the silver ions 24 growingaround them, so that the final result is the electrically conductivepaths 26. The degree of concentration of silver particles 22 or silverions 24 forming the electrically conductive paths 26 can be adjusted viathe duration or the intensity of the UV light 30. Because of theaccumulation of silver ions 24 at electrically conductive paths 26,these ions are simultaneously thinned out in the regions of the basematerial 18 located between two adjacent electrically conductive paths,so that a good insulation ratio exists between the adjacent paths 26.Once the UV light 30 is turned off and the mask 28 is removed, the basematerial 18 cools down, so that the liquefied regions harden, and thusthe conductive paths 26 formed remain in their positions.

A further advantageous example for attaining anisotropically conductivepaths in an adhesive will be described below. To the extent thatappeared useful for comprehension, reference numerals of FIGS. 1 and 2were used again, although here the production process proceedsdifferently.

First, the base material 18 is synthesized. To that end, 200 ml of 100%purified ethanol is heated to boiling while stirring. A mixture of 69.63g (=27.5 mol %) dichlorodiphenylsilane, 98.75 g (=70 mol %)dichloromethylvinylsilane and 5.20 g (=2.5 mol %) tetraethoxysilane isslowly added drop by drop. After this addition has been completed, themixture is boiled in reflux for 2 h; the HCl gas liberated in thereaction escapes via the reflux cooler and is carried through a hoseinto a container with water. The reaction mixture takes on a slightlyyellow color during the reaction. By distilling the solvent off in awater jet vacuum (30 mbar) at 70° C., a slightly viscous liquid isobtained. This precondensate is then repeatedly received in 200 ml ofethanol and reconcentrated at 70° C. and 30 mbar until it exhibits aneutral reaction with respect to the pH value. To the clear yellowishsolution that remains, the same quantity by volume of acetone is added,and the solution is heated to boiling and then quickly mixed with 73.8ml of 0.1 normal aqueous hydrochloric acid (=4.0 mols of water). Thesolution is then boiled in reflux for 3 h, whereupon the condensatesettling out causes a white cloudiness. After the solvent is extractedin a water jet vacuum at 70° C., a hydrolyzate is obtained, which isthen dissolved twice in the same quantity of acetone and each time isagain concentrated at 30 mbar and 30° C. The viscous, milky residue thatremains is then heated in a water jet vacuum at 180° C. until such timeas the following extinction conditions between the OH bands at 3620 cm⁻¹and 3400 cm³¹ 1 and the phenyl-CH bands at 3070 cm³¹ 1 result in the IRspectrum of the specimen:

    E.sub.3620 /E.sub.3070 =0.358±0.03

and

    E.sub.3400 /E.sub.3070 =0.624±0.06.

The result obtained is an Ormocer resin on the basis ofdiphenyldichlorosilane, methylvinyldichlorosilane, andtetraethoxysilane.

The thus-obtained matrix of the base material 18 is then synthesizedinto the complete adhesive system having the conductive particles 20. Tothat end, 2.3633 g of the previously synthesized Ormocer resin areaminofunctionalized with 0.0128 g (0.00529 mol) of AMDES(aminopropylmethyldiethoxysilane) for 30 minutes at 140° C. and thencooled down to 100° C. Next, 2.9107 g (0.0174 mol) of silver acetate aredissolved in 5 ml of acetone and mixed with 2.8958 g (0.0173 mol) ofDIAMO. This silver salt solution is added to the aminofunctionalizedOrmocer resin, and the mixture is stirred for 5 to 10 minutes at 100°C., until the viscosity increases sharply. Then, 0.0257 g (0.0105 epoxyresin equivalent) with Gy266 (produced by Araldit) are added drop bydrop, having previously been dissolved in 1.5195 g of acetone. Now at100° C. while stirring, the reaction is maintained until the desiredviscosity, which is determined by the "B-stage" test, is attained.

After the conclusion of the reaction, a base material 18 is obtainedthat is filled with approximately 30% silver in proportion by mass. Thebase material 18, provided with the silver as conductive particles 20,can now be applied in the layer 16 to a substrate 10. The layerthickness is 10 μm, for instance. The application of the layer 16 can bedone by conventional, well-known methods, such as blade coating.

Next, the layer 16 is subjected to a laser treatment. To that end, anargon ion laser with a laser spot diameter of 60 μm, operated in the VISrange and the multiline mode, for instance, is used. The laser power canbe varied depending on the type of anisotropically electricallyconductive paths 26 to be made. If conductor tracks are to be made, forinstance, the power may be 1/3 W. The laser is passed over the layer 16by the so-called "direct laser writing" method. The substrate 10, alongwith the layer 16, can be moved past the laser spot. To that end, thesubstrate 10 is disposed on a suitable X table, for instance. Theadvancement speed is 0.5 mm/sec, for instance. In accordance with theintended motion of the substrate 10, a conductor track is purposefullystructured in the layer 16 in those regions where the laser treatmenttakes place. During the laser treatment, a purposeful or in other wordslocal heating of the layer 16 occurs, in which the stabilization of thesilver is undone. This causes thermal destruction of silver complexes,for instance. The consequence is increased mobility of the silver ionsin certain regions. At the center of the interaction, silver accumulatesand is reduced purposefully. This creates a diffusion sink for thesilver ions. The regions with silver in the center grow to form theelectrically conductive paths 26, while around these regions a depletionof silver ions occurs. The purposeful reduction of the silver ispromoted by products of decomposition of the base material 16 arisingduring the local heating of the base material 16, or is reinforced bycomplexing agents of the stabilized silver. These agents act as reducingagents for the reduction of the silver.

In another example, the laser spot of the laser can be focused at aspecific site of the layer 16. This selected site is irradiated for acertain period of time, such as 10 seconds, with the laser power being 1W, for instance. As a consequence of the irradiation for the selectedlength of time, what is created at this site is a silver pitch, or inother words a single, defined electrically conductive path 26. The samechemical processes proceed within the layer 16 that lead to thepurposeful reduction of the silver in the irradiated region, so thatsilver colloids are created that establish the electrically conductiveconnection. In the unirradiated regions, the silver remains stable, sothat an isotropic conductivity is precluded by the distribution of thesilver in the base material 16 below the percolation threshold.Moreover, the silver ions in the immediate vicinity of the silver pitchcreated are depleted, so that the anisotropy of the conductivity isthereby improved.

Overall, it becomes clear that by suitable complexing of the basematerial 18, a very high degree of filling with silver can be attained.In the example shown, a mass proportion of 30% has been mentioned, butit can also be substantially higher, for instance 70%. By means of thesynthesis of the base material 18, precipitation out of the silver isprevented. At the same time, during the complexing, substances can beincorporated that enable or promote the ensuing reduction of the silverunder the influence of an exposure to light and/or a laser treatment.

From FIG. 3, a connection by means of the hot-melt adhesive layer 16 isillustrated; it may be produced on the basis of the UV exposure and/orthe laser treatment. Identical elements to those of FIGS. 1 and 2 areprovided with the same reference numerals and not explained again below.

In the example shown, the substrate 10 is glued to a further substrate34 via the hot-melt adhesive layer 16. The substrate 34 is likewisechosen merely as an example and may for instance be a chip thatintegrated electronic components. The substrate 34 has contact sites 36,which are meant to be electrically conductively connected to the contactsites 14 of the substrate 10. The substrate 34 is positioned above thesubstrate 10, with its contact sites 36 oriented in the direction of thehot-melt adhesive layer 16. The positioning is done such that thecontact sites 14 and 36 to be contacted face one another. Since theconnection grid of substrates 10 and 34 to be conducted is known per se,it is assured via the layout of the mass 28 that an electricallyconductive path 26 is present between each of the contact sites 14 and36 to be contacted. The substrates 10 and 34 are now joined together byheating of the hot-melt adhesive layer 16. This heating softens the basematerial 18, and the heating temperature is adjustable depending on thematerial of the base material 18. If the base material 18 issiloxane-based, for instance, the substrates 10 and 34 may for instancebe glued together at a temperature of about 120° C. The softened basematerial 18 then takes on the task of large-area adhesion promotionbetween the substrates 10 and 34, while the electrically conductiveconnection is effected via the contact sites 14, the paths 26 and thecontact sites 36. The resultant composite system is then heat-treated,so that the hot-melt adhesive layer 16 is cured. This can take place forinstance at a temperature of about 120° C., over a relatively longperiod of time, or at a higher temperature in a shorter period of time.The curing temperature may be chosen as a function of the temperatureresistance of the substrates 10 and 34.

With the anisotropically conducting adhesive of the invention, a systemis created that can be processed and manipulated easily. Especiallybecause conductive particles are extended out of the regions between twoconductive paths 26, the anisotropy of the hot-melt adhesive layer 16 issubstantially improved. The resultant conductive paths 26 aredistinguished by improved electrical conductivity and current-carryingcapacity, compared with known anisotropic adhesives. The conductivepaths 6 may be structured at arbitrary sites of a large-area existinghot-melt adhesive layer 18, so that in a simple technical process, anyarbitrary configuration of an electrically conductive connection betweentwo substrates can be realized. Moreover, with the anisotropicallyconducting adhesive of the invention, heavy-duty connections between thesubstrates 10 and 34 to be glued together can be made. As a result ofthe full-surface mechanical fixation of the substrates 10 and 34 overthe base material 18, the electrically conductive connections aresubject to only slight mechanical strain, thus improving the contactreliability of the entire arrangement. Any strains that might occur canbe absorbed by the elasticity of the base material 18, so that differentthermal expansions on the part of the substrate 10 or the substrate 34,for instance, can be compensated for without worsening the contactreliability.

We claim:
 1. An anisotropically conducting adhesive, comprising:a thermoplastic base material; and particles which include metal particles and metal ions, which are electrically conductive, and which are finely distributed within the thermoplastic base material below a percolation threshold, wherein the particles are enriched in certain regions under the influence of exposure to at least one of light and heat.
 2. The anisotropically conducting adhesive of claim 1, wherein the metal particles are colloidal silver particles and wherein the metal ions are silver ions.
 3. The anisotropically conducting adhesive of claim 2, wherein the colloidal silver particles have a particle size in the nm range.
 4. The anisotropically conducting adhesive of claim 2, wherein the colloidal silver particles are agglomerated and have a particle size ranging from 5 to 50 μm.
 5. A process for producing an anisotropically conducting adhesive, comprising:providing a material comprised of thermoplastic base material in which electrically conductive particles including metal particles and metal ions are dispersed; and exposing the material to at least one of light and heat in predetermined regions in a targeted manner so that a targeted local heating occurs in the exposed regions and an increased mobility of the metal particles and metal ions occurs which is effective to provide a plurality of anisotropically electrically conductive paths having an enriched amount of the electrically conductive particles compared to that of adjacent regions, wherein the metal ions contribute to the formation of the plurality of anisotropically electrically conductive paths by undergoing reduction from the metal ion to the metal.
 6. The process of claim 5, wherein the metal particles are colloidal metal particles.
 7. The process of claim 6, wherein the colloidal metal particles are colloidal silver particles, and wherein the metal ions are silver ions.
 8. The process of claim 5, wherein the metal particles which are dispersed in the thermoplastic base material are converted into colloidal metal particles by exposure to at least one of light and heat.
 9. The process of claim 8, wherein the colloidal metal particles are colloidal silver particles, and wherein the metal ions are silver ions.
 10. The process of claim 5, wherein reduction of the metal ions from metal ion to metal is performed purposefully in the respective regions in which the plurality of anisotropically electrically conductive paths are created.
 11. The process of claim 10, wherein reduction of the metal ions from metal ion to metal is performed purposefully by at least one of (a) including in the material a reducing agent effective for the reduction of the metal ions, (b) creating a reducing agent effective for the reduction of the metal ions by decomposing the material to provide a decomposition product which is said reducing agent effective for the reduction of the metal ions, and (c) creating a reducing agent effective for the reduction of the metal ions by including a complexing agent in the material and by decomposing the complexing agent to provide a decomposition product which is said reducing agent effective for the reduction of the metal ions.
 12. The process of claim 5, wherein the reduction from the metal ion to the metal is performed by means of localized treatment with a laser.
 13. The process of claim 5, wherein the electrically conductive particles are one of partially stabilized or fully stabilized by including in the material an agent effective to stabilize the electrically conductive particles.
 14. The process of claim 5, wherein the material is exposed to light through a mask having defined therein openings arranged in a predetermined layout.
 15. The process of claim 5, wherein the material is exposed to light, and wherein the light is absorbed by the metal particles.
 16. The process of claim 5, wherein the metal ions are colloidal metal ions, wherein the material is exposed to light, and wherein the light is absorbed by the colloidal metal ions.
 17. The process of claim 5, wherein the material is exposed to light, and wherein the light is UV light.
 18. The process of claim 5, wherein the plurality of anisotropically electrically conductive paths having an enriched amount of the electrically conductive particles compared to that of adjacent regions has a degree of enrichment, and wherein the degree of enrichment is adjustable by varying duration or intensity of exposure to the at least one of light and heat. 